It is quite known that an improvement of the impact resistance properties of a relatively fragile (brittle) thermoplastic material such as styrene/acrylonitrile copolymer (SAN), may be obtained by the addition to the SAN of a gummy polymer with a sufficiently high degree of chemical/physical compatibility with SAN.
As a rule, the compatibility of the rubber with SAN is achieved by grafting onto the rubber segments of styrene/acrylonitrile copolymer. The rubbers used for such a grafting may be of different types. Those normally used are: polybutadiene, ethyl- and/or butyl polyacrylate, the ethylene/propylene/diene terpolymers. The graft on the rubber may be carried out in various different ways. Thus, for instance, one may operate on the rubber in: emulsion, suspension or in solution, by the addition of a radical catalyst and of a mix or blend of styrene or acrylonitrile monomers. At any rate, the modification of the rubber requires a special set up in which to start the grafting. The grafted rubber must then undergo some processes of separation from the residual monomers, of purification and drying before they may be admixed to the SAN in the desired ratios.
The rigid matrix SAN, in general is synthetized separately. In some instances said matrix may be prepared in the same reactor in which is carried out the grafting of the rubber. In this case the process is however made even more complicated by the higher number of variables to be controlled.